Sexy scents

Do humans give off pheromones?

With expert information from Dr Tristram Wyatt, University of Oxford, UK

She hadn’t been wearing perfume that day, but Joe had gone wild over her natural aroma. He’d often made comments about Kayleigh wafting her irresistible pheromones – the invisible chemical signals released to signal sexual attraction found in many animals – in his direction. Was she enticing him with her natural chemical secretions? She did some googling but couldn’t find anything definitive.

One name kept cropping up in her research… Dr Tristram Wyatt, Senior Research Fellow, Department of Zoology at the University of Oxford, UK. She tracked him down so that he could explain attraction through smell.

Kayleigh Daniels: It doesn’t look like any human pheromones have been discovered yet, so why do you think that people believe they exist?

Dr Tristram Wyatt: The idea that humans might have them is a powerful and attractive one – people want to believe there are pheromones that could make us irresistible. And this belief is encouraged by the many commercial websites that advertise ‘pre-bottled human pheromones’. Their sales pitch, complete with photos of people looking ‘sciencey’ in white lab coats, is based on highly dubious claims made by some US professors in the 1990s.[1]

They claimed that two molecules, called ‘androstadienone’ and ‘estratetraenol’, were human pheromones. But no evidence was ever provided, and none has been found since. These professors created a scientific myth. Since then, other scientists have tested the molecules for mostly psychological effects with an uncritical eye – and these tests do not provide evidence that these molecules are pheromones.

KD: OK. So, why do we wear perfume to make ourselves more attractive?

Dr TW: Perfumes both mask our own body smells and add new attractive smells. They have a long history, supported by the discovery of perfume bottles used by the ancient Egyptian pharaohs. Some of the molecules used in many perfumes, such as the musks, are originally from the sexual secretions of other mammals.

KD: Oh, really!

Dr TW: Yes, though synthetic ones are mostly used now. But perfumes are much more a product of fashion and advertising than biology. We can be pretty sure of this, in the way that the popularity of different perfume brands changes over time. Your grandparents chose different perfumes to your parents, and you would choose different ones today. Perfumes certainly make us feel good, give confidence, and, used in moderation, they add to allure.


KD: 
Right. So, why does sex smell like it does, if we’re not actually giving off pheromones?

Dr TW: The science part goes like this… sexual arousal releases the hormone adrenaline, which activates the apocrine glands – the ‘smelly’ sweat glands. There are lots of these in the armpits, genitalia, and other parts of the body such as around the nipples. Some of the molecules secreted in your armpits during sexual arousal are the ones you’ll recognise from other emotional moments which cause release of adrenaline, including anxiety and pain. We don’t actually know that much about all the smelly molecules that are released during sex. But we know that there are also secretions from the foreskin of men (if uncircumcised) and vaginal secretions from women.

KD: And what about that particular smell?

Dr TW: Which particular smell?

KD: *clears throat*

Dr TW: Semen?

KD: Exactly.

Dr TW: The distinctive smell of semen comes from molecules including putrescene, which causes the slight rotting meat smell, and its breakdown products, the appropriately named ‘spermidine’ and ‘spermine’.[2]

KD: If I think back to the last time there were quite a few different aromas…

Dr TW: Yes, sexual activity creates lots of different types of odours; there are musky smells and sweaty ones. The strength of the smells may depend on how recently you and/or they washed.

KD: My internet searches kept bringing up this phrase ‘immuno-compatibility’ and how it’s actually this ‘sciencey’ element that guides us to our eventual partners.

Dr TW: In the 1990s, the notion that differences in immune system genetics could be influence partner choice in humans proliferated within certain areas of the scientific community. The theory was that the smell of people with immune systems that differed to yours, were more appealing than those with genetically similar immune systems. Mating with someone with genes the most genetically different to yours would create more genetically diverse offspring, and would therefore be healthier.

Studies had previously been carried out on mice. In mice, it was shown that they preferred the smell of those that had different immune system genes. In human tests involving Swiss students, subjects slept in a T-shirt to capture their natural odour-print. They were also tissue-typed in the same way that organ donors/recipients are. From the immune system tissue-typing, it was shown that like mice, the students preferred the smell of the person of the opposite sex that were more different at the immune system genes. However, the results from the Swiss experiments have not always been easy to repeat. [3]

KD: Not least because of their blatant heteronormativity…

Dr TW: So, what about human mate choice ‘in the wild’? A study coming out soon shows no such effects from some 250 Dutch couples. No evidence of choice for difference in immune system genes was found…

So although Kayleigh was now more clued-up up on the power of smell, she still had basically no idea why she and Joe were such a match. But since they were having so much fun, she’d keep seeing him…

References:

[1] Wyatt T, ‘Sexing up the human pheromone story: How a corporation started a scientific myth’, The Guardian, 4 March 2015

[2] Sanford K, ‘Spermine and spermidine podcast’, chemistyworld.com, 15 March 2017

[3] Wyatt T, quoted in ‘The Dubious Science of Genetics-Based Dating’, The Smithsonian, 14 February 2018